Configure Dependabot NuGet update groups

[Copilot]

Dependabot was configured to check NuGet updates weekly, but every dependency update surfaced as an individual PR. This change introduces grouped update rules so related packages are updated together and dependency maintenance is easier to triage.

• Dependabot grouping for NuGet

  • Added groups to the existing NuGet updater in .github/dependabot.yml.
  • Grouped related dependencies into focused buckets (runtime/hosting, OpenXML/rendering stack, test tooling, and general tooling) using package name patterns.

• No scope expansion

  • Kept the existing updater scope intact (package-ecosystem: nuget, directory: /, weekly cadence).
  • Only adjusted grouping behavior; no new ecosystems or schedules were introduced.

version: 2
updates:
  - package-ecosystem: "nuget"
    directory: "/"
    schedule:
      interval: "weekly"
    groups:
      rendering:
        patterns:
          - "DocumentFormat.OpenXml"
          - "DiagramForge"
          - "Markdig"
          - "TextMateSharp*"
      testing:
        patterns:
          - "xunit*"

[Copilot]

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